1. Technical Filed
This document relates to gas separation membranes.
2. Background
Membrane gas separations have been increasing in importance since the 1980s. One application of particular interest is the removal of CO2 from natural gas. Many natural gas sources are too high in CO2 to be allowed into the natural gas pipeline grid.
The most common membrane used in removal of CO2 from natural gas is cellulose triacetate (CTA) formed by the immersion precipitation process (U.S. Pat. No. 4,243,701, Riley et al. 1981). This membrane has permeability to CO2 about 15 times higher than that to methane. When natural gas is pressurized against one side of the membrane, CO2 preferentially passes through the membrane and the retentate gas is purified. A membrane separation of the permeate gas can recapture essentially all the methane.
An issue with CTA membranes for gas separations is the membranes are only pliable in the wet state. For CTA to be used in gas separations, after rolling into an element, the membrane must be dried in a two-step process. After drying the membrane becomes brittle and many failures in the field are due to membrane cracking
There are membranes made from other cellulose esters that are more rubbery and pliable when dried. Cellulose acetate butyrate (CAB) has been shown to be more rugged in gas separations but its performance is lower than that of CTA and it is not used commercially.
Regardless, any conventional two-layer membranes in use are all made by applying a coating to an existing membrane.